Pyrotechnic delay composition

ABSTRACT

A pyrotechnic delay composition comprising at least one oxidant and at least one reducing agent. This composition comprises tungsten as the first reducing agent, a first oxidant consisting of triiron tetraoxide, and a second oxidant having an oxygen content greater than that of the first oxidant.

The technical field of the invention is that of pyrotechnic delaycompositions.

Such compositions are well known. They are implemented in the form ofrigid delay columns or soft cords in a metallic sheath and allow totrigger a pyrotechnic event at the end of a specified time interval.

They are used, for example, in the munitions field to causeself-destruction of munitions that have not reached their target. Theyare also used in the space field in the form of delay cords betweenvarious components of a ballistic launcher.

Many pyrotechnic compositions have been developed in the past.

The most commonly known compositions combine tungsten, barium chromateand potassium perchlorate. It is also possible to incorporate ultrafinesilica in these compositions, as described in patent FR 2 706 449.

These compositions have the advantage of generating little gas and maybe shaped as cords of reduced diameter (sheath diameter of less than 4mm), without the risk of the cord bursting or the combustion beingstopped.

However, they use a chromium-based compound (barium chromate—BaCrO₄) forwhich the toxicity of chromium leads to its prohibition (REACH EuropeanDirectives).

It is therefore necessary to define a less toxic delay composition butone that offers the same properties as those of the known composition:reduced combustion rate (less than 4 mm/s in a flexible sheath of smalldiameter (less than 3 mm)), absence of gas, reliability of combustionfor the entire temperature range that is required for military and spaceapplications (−40° C. to +60° C.).

It has been proposed in the patent WO2017/084916 that this compositionshould be replaced by a new delay composition associating one or moreiron oxides with zirconium or zirconium hydride. However, the newcomposition is likely to generate hot spots in the reaction zone and,according to this patent, it is necessary to add one or more additivesgenerating slag in order to mitigate the reaction.

This results in a complex pyrotechnic composition, comprising manycomponents whose dosage is difficult.

It is the object of this invention to provide a pyrotechnic delaycomposition devoid of chromium compounds, thus complying with the REACHdirectives, wherein this delay composition has a formulation thatensures reliable operation without stopping combustion in the case ofreduced diameter.

More particularly, this composition makes it possible to produce delaycords comprising a sheath based on tin and bismuth, cords whose outerdiameter of the sheath is between 7.2 mm and 2.6 mm and which may, by achoice of specific formulation, ensure a burning rate of 2.64 mm/s to4.0 mm/s.

Thus, the object of the invention is a pyrotechnic delay compositioncomprising at least one oxidant and at least one reducing agent, whereinthe composition is characterized in that it comprises tungsten as thefirst reducing agent, a first oxidant constituted by triiron tetraoxide,and a second oxidant having an oxygen content higher than that of thefirst oxidant.

Triiron tetraoxide is a well-known iron oxide found naturally in theform of ore known as magnetite. It is a non-toxic and inexpensivematerial. It has the disadvantage of being a relatively weak oxidant. Infact the mass of oxygen that it can release per mass of material is ofthe order of 28%.

The risk with such a material is therefore the interruption of thecombustion of the delay composition.

By way of comparison, the barium chromate of the known delay compositionmay release 63% of its oxygen mass.

The patent WO2017/084916 overcomes this disadvantage by combining theiron oxide with a strong reducing agent such as zirconium. However, thereaction then leads to the production of metal remaining incandescent,while the zirconium reacts strongly with the reduced iron resulting inan intermetallic type reaction. It then becomes necessary to mitigatethe combustion with slag generating materials.

On the contrary, the invention proposes to conserve tungsten as areducing agent. In fact, its reduction capacity (3.98 grams of reducingagent for one gram of oxygen) is good although lower than that ofzirconium, while it also has a high thermal conductivity (174 Wm⁻¹K¹)which allows it to dissipate calories and reduce the burning rate of thecomposition by draining the heat of the reaction.

In order to avoid stopping the combustion of the delay composition, thetriiron tetraoxide is associated with a second oxidant which has anoxygen content greater than that of the triiron tetraoxide.

This second oxidant makes it possible to overcome the weaknesses of thetriiron tetraoxide.

For example, one of the following oxidants may be chosen: potassiumperchlorate, barium peroxide, bismuth oxide, diodine pentoxide . . . .

Potassium perchlorate is preferably chosen as the second oxidant. Infact, the oxygen content of potassium perchlorate is 46.2%, which isrelatively high. It is therefore sufficient to reduce the amount of thisoxidant to maintain a uniform combustion front.

Advantageously, the composition according to the invention may comprisea second reducing agent constituted by aluminum.

This addition of aluminum makes it possible to correct the burning rateof the delay composition.

The delay composition will therefore comprise: from 20% to 60% by weightof tungsten, and from 30% to 70% by weight of triiron tetraoxide.

More specifically, the delay composition according to the invention willhave the following composition:

20% to 60% by weight of tungsten,

30% to 70% by weight of triiron tetraoxide,

1% to 30% by weight of potassium perchlorate,

0% to 10% by weight of aluminum.

In a preferred manner, the delay composition according to the inventionwill have the following composition:

30% to 40% by weight of tungsten,

45% to 55% by weight of triiron tetraoxide,

5% to 15% by weight of potassium perchlorate,

0% to 10% by weight of aluminum.

The examples of compositions which will now be described, make itpossible in a nonlimiting manner to demonstrate other advantages of thecomposition according to the invention.

The selected tungsten has a particle size of between 0.2 mm and 0.315 mmin all the compositions produced.

The particle size of the triiron tetraoxide is between 0.2 mm and 0.315mm.

That of potassium perchlorate is between 0.2 mm and 0.315 mm.

That of aluminum is between 0.2 mm and 0.315 mm.

A first series of tests was carried out with delay compositions in theform of cords in a tin sheath.

The materials are dry-blended and compressed into a ductile tin metaltube of 17 mm initial outside diameter, and then the loaded tube isprogressively reduced in diameter by being passed through dies to obtaina 3 mm outer diameter of the delay cord.

An ignition composition is then placed at the inlet of the tube that canignite the delay composition (for example a conventional compositioncombining titanium and carbon).

Several tubes having a length of 300 mm are thus produced. The burningtime of each loaded tube of delay composition is then measured.

The table below lists various compositions that were prepared and whoseburning rate was measured. The percentages that are given relate to thetotal mass of the composition.

Triiron Potassium Burning Tungsten W Aluminum tetraoxide perchloraterate (%) Al (%) Fe₃O₄ (%) KClO₄ (%) (mm/s) Example 1 39 / 50 11 4.44Example 2 31 8 50 11 4.35 Example 3 25 14 50 11 4.17 Example 4 25 8 5611 3.45

Note that the relative increase in the mass of triiron tetraoxidereduces the rate of combustion (Example 4).

By containment in a sheath, aluminum decreases the burning rate of thedelay composition.

Other tests have verified that, if the potassium perchlorate is omitted,ignition of the delay composition cannot be effected with the chosenignition composition (titanium/carbon). But other ignition compositionsmay be considered.

Another series of tests was carried out with other compositions in whichthe level of aluminum was varied.

These tests were carried out in an ignition configuration of thecomposition on a gutter, and not with the confinement of a cord.

The results are summarized in the following table:

Triiron Potassium Burning Tungsten W Aluminum tetraoxide perchloraterate (%) Al (%) Fe₃O₄ (%) KClO₄ (%) (mm/s) Example 5 38 1 50 11 0.87Example 6 35 4 50 11 1.01

Note the accelerating effect of combustion provided by aluminum in thisunconfined combustion configuration.

These latter compositions could also be formed into tin sheaths. Thecombustion rates would then be higher (of the order of 3.5 mm/s to 4.5mm/s for a tin sheath with an external diameter 3.1 mm).

1. Pyrotechnic delay composition comprising at least one oxidant and atleast one reducing agent, wherein the pyrotechnic delay compositioncomprises tungsten as the first reducing agent, a first oxidantconsisting of triiron tetraoxide and a second oxidant having an oxygencontent greater than that of the first oxidant.
 2. Pyrotechnic delaycomposition according to claim 1, wherein the second oxidant is selectedfrom among the following oxidants: potassium perchlorate, bariumperoxide, bismuth oxide, diiodine pentoxide.
 3. Pyrotechnic delaycomposition according to claim 1, wherein the pyrotechnic delaycomposition comprises a second reducing agent consisting of aluminum. 4.Pyrotechnic delay composition according to claim 1, wherein thepyrotechnic delay composition comprises: from 20% to 60% by weight oftungsten, and from 30% to 70% by weight of triiron tetraoxide. 5.Pyrotechnic delay composition according to claim 4, wherein thepyrotechnic delay composition comprises: 20% to 60% by weight oftungsten, 30% to 70% by weight of triiron tetraoxide, 1% to 30% byweight of potassium perchlorate, 0% to 14% by weight of aluminum. 6.Pyrotechnic delay composition according to claim 5, wherein thepyrotechnic delay composition comprises: 30% to 40% by weight oftungsten, 45% to 55% by weight of triiron tetraoxide, 5% to 15% byweight of potassium perchlorate, 0% to 14% by weight of aluminum. 7.Pyrotechnic delay composition according to claim 6, wherein thepyrotechnic delay composition comprises: 39% by weight of tungsten, 50%by mass of triiron tetraoxide, 11% by weight of potassium perchlorate.8. Pyrotechnic delay composition according to claim 6, wherein thepyrotechnic delay composition comprises: 31% by weight of tungsten, 8%by weight of aluminum, 50% by weight of triiron tetraoxide, 11% byweight of potassium perchlorate.
 9. Pyrotechnic delay compositionaccording to claim 5, wherein the pyrotechnic delay compositioncomprises: 25% by weight of tungsten, 14% by weight of aluminum, 50% byweight of triiron tetraoxide, 11% by weight of potassium perchlorate.10. Pyrotechnic delay composition according to claim 5, wherein thepyrotechnic delay composition comprises: 25% by weight of tungsten, 8%by weight of aluminum, 56% by weight of triiron tetraoxide, 11% byweight of potassium perchlorate.
 11. Pyrotechnic delay compositionaccording to claim 6, wherein the pyrotechnic delay compositioncomprises: 38% by weight of tungsten, 1% by weight of aluminum, 50% byweight of triiron tetraoxide, 11% by weight of potassium perchlorate.12. Pyrotechnic delay composition according to claim 6, wherein thepyrotechnic delay composition comprises: 35% by weight of tungsten, 4%by weight of aluminum, 50% by weight of triiron tetraoxide, 11% byweight of potassium perchlorate.